This invention relates generally to temperature sensing devices, and, more particularly to a temperature sensing device which is capable of providing substantially instantaneous response to temperature changes.
There are many requirements for a temperature sensing device which is capable of substantially instantaneously providing an indication of temperature changes. For example, the operation of many currently available jet engines is directly linked to variations in temperatures in various parts of the engine. In the F101 jet engine, the entire control thereof is accomplished by the engine automatically responding to the pilot's command and specific parameters measured by the engine control system.
The engine control system regulates three engine parameters which are influenced by the temperature of the air entering the compressor of the engine. These parameters are engine speed, compressor variable stator vane position and acceleration fuel flow. Each of these three fundamental engine parameters is greatly affected by compressor inlet temperature. If, during engine transient operation the temperature sensor cannot vary its temperature signal output as rapidly as the actual temperature variances, the control system of the engine will incorrectly schedule the three parameters noted above.
The effect of an error in sensed compressor inlet temperature has the greatest impact upon compressor stator vane scheduling and thus the stall margin of the compressor. For example, at a mid range operating point, a 10.degree. F. sensed error can result in the stators being 0.8 degrees off schedule. This is 50% of the entire tolerance band for the entire control system and engine. Thus it is evident that if the air temperature is changing at a rate of 100.degree. F. per second, the temperature sensor must be extremely quick reacting or the engine must be very tolerant of scheduling errors. For high performance aircraft, however, neither of the above statements is true.
The speed at which a temperature sensor can sense a change in air temperature is generally proportional to the density of the air passing over the sensor. The response of the sensor to a step change in air pressure is called the "time constant". This means that with an instantaneous change in temperature (a step input), the sensor output will be approximately 63% of the change in one time constant. At a typical idle condition (20 lb/sec/sq ft) it takes one second for the sensor to respond to only 63% of the real temperature change. Consequently, this is a major limiting feature in the acceleration of a jet engine. Therefore, with a sensor capable of reacting almost instantaneous with a temperature change, the jet engine could accelerate substantially faster.
It is therefore apparent that a temperature sensing device capable of substantially instantaneously providing an output indicative of temperature changes would be extremely beneficial in increasing the efficiency of jet engines or, for that matter, any other device which is dependent upon such temperature changes for its operation. Unfortunately, such sensors are currently unavailable since not only must these sensors respond rapidly to temperature changes, they must also (1) be able to mechanically survive the jet engine operating environment; (2) sense the steady state temperature within the accuracy required for the control system of the jet engine (this requirement is 1.0% per degree Rankine; and (3) respond to a change in compressor inlet temperature rapidly enough such that the resulting errors in sensor output do not result in engine parameter scheduling errors which are detrimental to the engine operation. In other words, such a sensor must be strong, accurate and fast. Unfortunately, from a design and mechanization standpoint, the requirement that the sensor be strong is in opposition to the requirement that the sensor be fast; that is, a large mass takes substantially more heat (BTU's) to change its temperature than a small mass.